Often, slags or other metallic residues produced in smelting operations contain molybdenum in concentrations sufficient to provide an economic incentive for its recovery. This is particularly true of various copper smelting slags. Generally, the molybdenum present in the slag is due to inefficiencies in the flotation step for separating molybdenite from copper sulfide minerals prior to the smelting step. (Other slags such as steel slags may also contain molybdenum.)
Analysis of some typical copper reverberatory slags shows that the molybdenum is dissolved throughout the glassy iron silicate phase. Standard selective leaching processes cannot be used to recover and isolate the metal values in these slags because the iron silicate is inert to dilute acids and caustic solutions. Concentrated caustics and acids will dissolve the slag with an uneconomical consumption of reagent.
An extraction process to concentrate molybdenum from slags has been described in U.S. Pat. No. 3,857,699 to Paul R. Ammann et al. entitled "Process for Recovering Non-Ferrous Metal Values from Reverberatory Furnace Slags" the teachings of which are incorporated herein by reference. An important step in that process utilizes the preferential solubility of molybdenum in an iron-sulfur matte. Thus, the molybdenum in the molten slag is extracted into a molten iron sulfide matte from which it is recovered.
As is taught in that patent, after oxidizing (roasting) the matte, final recovery of the molybdenum is accomplished hydrometallurgically by leaching the matte to produce a pregnant liquor and then selectively extracting the molybdenum values from the pregnant liquor.
A major disadvantage of hydrometallurgical recovery processes is that the cost per pound of molybdenum becomes high when the matte or alloy contains low concentrations of molybdenum (e.g., less than 10%).